Silyl-containing thermosetting networks and methods of degradation

ABSTRACT

A process of: providing a polyfunctional compound selected from polyisocyanate, polyacrylate, and polyepoxy; and reacting the polyfunctional compound with a hydroxyl- or amine-terminated silyl-containing compound. The polyfunctional compound and the silyl-containing compound are at least difunctional. A thermoset made by this process.

This application is a divisional application of U.S. application Ser.No. 15/843,181, filed on Dec. 15, 2017, which claims the benefit of U.S.Provisional Application No. 62/434,628, filed on Dec. 15, 2016. Theprovisional application and all other publications and patent documentsreferred to throughout this nonprovisional application are incorporatedherein by reference.

TECHNICAL FIELD

The present disclosure is generally related to silyl-containingthermosets.

DESCRIPTION OF RELATED ART

Thermosetting materials, commonly referred to as thermosets, are formedfrom the chemical reaction of two or more synthetic materials togenerate a permanent three-dimensional network. Unlike thermoplasticmaterials, thermosets cannot be melted and reflowed into another form,which makes them useful for applications where thermal, chemical and/orUV-oxidative resistance are required. However, the cross-linked networkof thermosets makes them difficult to degrade and destroy. Thermosetsoffer unique attributes and functional properties, which enables theirutilization in numerous military and commercial applications. Forexample, thermosets are used to provide high-performance topcoats andprimers for ships, aircraft and ground vehicles, adhesives for aircraftcanopies, high-strength composites for ship hulls and the fuselage ofaircraft, and high-temperature composites for aircraft exhausts.

Thermosets are currently degraded by using harsh chemicals,incineration, or physical methods. For instance, Navy aircraft topcoatsand primers are often removed with methylene chloride, which solvatesand swells the polymeric chains of the coatings so they can be easilyremoved via scraping or brushing. However, methylene chloride is highlytoxic and carcinogenic to humans, and less toxic alternatives, such asperoxides and benzyl alcohols, have proven to be less effective atcoating removal. Abrasive blasting is utilized to physically removecoatings, yet these methods present a health hazard due to thegeneration of microscopic particles. As a result, containment withsufficient ventilation, in addition to respirators, must be utilized byworkers to reduce health risks.

Thermosets with bonds that can be selectively cleaved have been recentlydeveloped to enable these materials to be more easily degraded anddestroyed. Examples include epoxies that contain alkene linkages, whichwere cleaved using harsh oxidizing reagents, such as permanganates, andepoxies that contain ester linkages, which were cleaved with strongacids or bases. However, in all cases, these reagents were slow tocleave the selective bonds in epoxy thermosets unless a large excess ofreagent and heat were applied. Furthermore, bond cleavage within theseepoxy thermosets was stoichiometric, meaning that only one bond wascleaved per chemical reagent, thus not sufficiently degrading thecross-linked system until a substantial number of bonds were broken.Conversely, thermoset components designed with an excess of selectivecleavable groups may lack sufficient properties for many of theaforementioned applications, even though they are easy to degrade. Todate, these limitations have prevented the development of durable,thermally stable, and rigid thermosets that can also be easily degradedand destroyed on-demand with mild and benign chemicals.

BRIEF SUMMARY

Disclosed herein are a process and a thermoset made thereby comprising:providing a polyfunctional compound selected from polyisocyanate,polyacrylate, and polyepoxy; and reacting the polyfunctional compoundwith a hydroxyl- or amine-terminated silyl-containing compound. Thepolyfunctional compound and the silyl-containing compound are at leastdifunctional.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation will be readily obtained by reference tothe following Description of the Example Embodiments and theaccompanying drawings.

FIG. 1 shows an example of a thermoset formed from a silyl-containinghydroxyl-functional molecule and a tri-functional aliphatic isocyanate.

FIG. 2 shows an example of cascading bond cleavage and release ofvolatile molecules within a polyurethane thermoset that has been treatedwith a fluoride salt stimulus.

FIG. 3 shows an example of a thermoset formed from a silyl-containingamine-functional molecule and a tri-functional aliphatic isocyanate.

FIG. 4 shows an example of a thermoset formed from a silyl-containingamine-functional molecule and a di-functional epoxy.

FIG. 5 shows silyl-centered diols, triols, and tetraols for formingthermosets that can disassemble.

FIG. 6 shows silyl-centered extended chain carbamates with hydroxyl andamine functionality.

FIG. 7 shows silyl-centered extended chain carbonates with hydroxyl andamine functionality.

FIG. 8 shows a clear silyl-containing polyurethane thermoset.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

In the following description, for purposes of explanation and notlimitation, specific details are set forth in order to provide athorough understanding of the present disclosure. However, it will beapparent to one skilled in the art that the present subject matter maybe practiced in other embodiments that depart from these specificdetails. In other instances, detailed descriptions of well-known methodsand devices are omitted to not obscure the present disclosure withunnecessary detail.

Described herein are hydroxyl and amine-terminated silyl-containingmolecules for forming thermosets that can disassemble on-demand whentreated with specific chemical reagents. The technology is based onhydroxyl- and amine-functional molecules that contain silyl groups andaliphatic linkages. The silyl group is defined as the “trigger”. Thesemolecules can be reacted with a cross-linker molecule, such as analiphatic polyisocyanate, to form a stable thermoset film with tailoredmechanical properties. FIG. 1 shows an example of a polyurethanethermoset formed from a silyl-containing hydroxyl-functional moleculeand a tri-functional aliphatic isocyanate.

Unlike traditional thermosets, these materials can be degraded intosmaller molecules by treating with a specific chemical “stimulus”, suchas mild and relatively benign fluoride salts. Addition of the stimulusresults in activation of the trigger via cleavage of the silicon-carbonbond, followed by subsequent cascading bond degradation and release ofsmaller molecules as the chains within the thermoset disassemble.Ethylene and carbon dioxide, which are volatile molecules, are releasedduring disassembly. FIG. 2 shows an example of bond cleavage within apolyurethane thermoset that has been treated with a fluoride saltstimulus.

Potential applications for these disassembling thermosets include: 1)rapid removal of aircraft coatings without utilizing toxic chemicals,such as methylene chloride, 2) selective removal of a topcoat from anunderlying organic composite without affecting the composite, 3)self-destructing electronics, 4) recyclable materials, and 5) rapidremoval of bandage adhesives.

The disclosed thermosets may be made by reacting a polyfunctionalcompound with a silyl-containing compound that possesses organicfunctional groups. Both compounds are at least difunctional. As usedherein, the term “thermoset” includes linear polymers made solely fromdifunctional compounds. Cross-linked thermosets may be made when eitheror both of the compounds are at least trifunctional, as shown in FIGS.1, 3, and 4. The polyfunctional compound is a polyisocyanate, apolyacrylic, or a polyepoxy. The silyl-containing compound has hydroxyland/or amino groups. The possible reactions between these functionalgroups are shown below.

—NCO+HO—→—NH—CO—O—

—NCO+NH₂—→—NH—CO—NH—

—C(CH₂)—CO—OH+HO—→—C(CH₂)—CO—O—

—C(CH₂)—CO—OH+NH₂—→—C(CH₂)—CO—NH—

—(C₂H₃O)+HO—→—CHOH—CH₂—O—

—(C₂H₃O)+NH₂—→—CHOH—CH₂—NH—

When a polyisocyanate is used, it may be aliphatic, aromatic,cycloaliphatic an isocyanate homopolymer, or an isocyanate-functionalpre-polymer. One suitable polyisocyanate is an aliphatic trimer based onhexamethylene diisocyanate (FIG. 1). Other examples include, but are notlimited to, toluene diisocyanate, methylene diphenyl diisocyanate,isophorone diisocyanate, isocyanate homopolymers, isocyanate-functionalurethanes, isocyanate-functional polyesters, isocyanate-functionalpolyethers, isocyanate-functional polysiloxanes, and mixtures thereof.Acrylate- and epoxy-functional molecules can also be used to form across-linked thermoset. Suitable epoxy- or acrylate-functional compoundsinclude, but are not limited to, epoxy-functional dimethylpolysiloxanes,epoxy-functional polydimethyldiphenylsiloxanes, aliphatic epoxies,aromatic epoxies, cycloaliphatic epoxies, acrylate-functionaldimethylpolysiloxanes, and 1,6-hexanedioldiacrylate.

Combinations or more than one polyisocyanate, polyacrylic, and/orpolyepoxy may be used.

One general formula for the silyl-containing compound is shown below. Xand X′ are derived from hydroxyl or amino groups. When m is 0, thecompound is a small molecule, and may be made by addition of hydroxyl oramino to a vinyl group by methods known in the art. Chain extension, asdescribed below, may be used to make the compound larger, resulting in apositive value for m. Combinations of more than one hydroxyl and/oramino compounds may be used, so that each X may be either —O— or —NR²—and each X′ may be either —OH or —NHR². The value of n is 0, 1, or 2,such that there are at least two terminal hydroxyl and/or amino groups.Each R¹ group is an alkyl or aryl group, each R² group is —H or an alkylor aryl group, and each R³ group is an alkylene group. Morespecifically, the silyl compound may have the formula SiR¹_(n)[(CH₂)₂OH]_(4-n).

SiR¹ _(n)[R³—(O—CO—X—R³)_(m)—X′]_(4-n);

Suitable silyl compounds include, but are not limited to,Si(CH₃)₂(CH₂CH₂OH)₂, Si(Ph)₂(CH₂CH₂OH)₂, SiPh(CH₂CH₂OH)₃, Si(CH₂CH₂OH)₄,Si(Ph)₂(CH₂CH₂OCON(CH₃)CH₂CH₂OH)₂, wherein Ph is a phenyl group. Whenthe R³ group is present, suitable alkylenes include, but are not limitedto, ethylene and propylene.

The silyl-containing hydroxyl and amine-functional molecules can possessa variety of compositions, sizes, and functionalities. For example, thesilyl group can contain dimethyl, diphenyl, or other combinations ofalkyl and aryl groups, whereas simple molecules can possess di-, tri-and tetra-hydroxyl functionality as shown in FIG. 5. These molecules canalso be used to synthesize chain-extended molecules that contain eithercarbamate and/or carbonate groups and terminal hydroxyl or amine groups.FIG. 6 shows examples of the extended chain carbamate-containingstructures that can be synthesized based on the molecules in FIG. 5,whereas FIG. 7 shows examples of the extended chain carbonate-containingstructures that can be synthesized based on the molecules in FIG. 5.Extended-chain molecules can also possess both carbamate and carbonatelinkages. The purpose of utilizing the extended chains is to provide forgreater disassembly within a thermoset, as the longer chains candisassemble into ethylene, carbon dioxide, and cyclic structures, suchand oxazolidinones and dioxolanones. Greater bond fragmentation hasshown to provide for faster thermoset disassembly when treated with thesame amount of a stimulus as those thermosets formed using molecules inFIG. 5.

In addition to the structures shown in FIGS. 3-5, the silyl-containingmolecules can possess hydrocarbon chains with lengths ranging from oneto many methylene linkages. The purpose of using different hydrocarbonchains is to enable tailored attributes, such as mechanical properties,viscosity, solubility, and rates of chain disassembly. Thesilyl-containing molecules can also possess hydrocarbon chains withbranched structures, polyether linkages, polyester linkages, orpolysulfide linkages, all to provide for tailored mechanical properties,viscosity, or solubility.

The two compounds may be reacted to form the thermoset according tomethods known in the art. The reactions may generally proceedspontaneously, however a catalyst may be used. A solvent, pigment, oradditive is also optional. Another optional component is a reactivediluent. The diluent need not contain a silyl group, but does have atleast one hydroxyl or amine group. Thus, the diluent may bemonofunctional.

The amount of the silyl-containing compound used in the reaction may beat least 5 wt. % of the total amount of the hydroxyl- oramine-terminated compound and the polyfunctional compound. As usedherein, this is equivalent to stating that the thermoset comprises atleast 5 wt. % of the silyl-containing compound.

The thermoset may be formed by applying the two compounds to a substratevia spray, brush, roll, print or dip method before the thermoset isformed. The compounds may be applied as mixture or as separateapplications, which may be simultaneous. Some reaction of the compoundsmay have occurred, as long as a cross-linked thermoset is not formeduntil the materials have been applied to the surface.

The thermoset can be disassembled by reaction with a fluoride salt, anacid, or a base to cleave the silicon-carbon bonds. Any degree ofcleavage may be performed, whether the thermoset remains intact or iscompletely degraded. Suitable compounds for cleaving the bonds include,but are not limited to, tetrabutylammonium fluoride, stannous fluoride,potassium fluoride, sodium fluoride, a Lewis acid, a Lewis base, aBronsted acid, or a Bronsted base.

The following examples are given to illustrate specific applications.These specific examples are not intended to limit the scope of thedisclosure in this application.

EXAMPLE 1

Synthesis of 2,2′-(dimethylsilanediyl)bis(ethan-1-ol)—A solution ofdimethyldivinylsilane (1.0 g, 8.91 mmol, Sigma-Aldrich) in 10 mL drytetrahydrofuran (THF) was added dropwise to a 0.5 M solution of9-borabicylco[3.3.1]nonane (9-BBN) in THF (17.82 mmol, 36 mL,Sigma-Aldrich) and the resulting mixture was stirred at room temperaturefor 4 hours. This was followed by the addition of water (10 mL) and 3Maqueous sodium hydroxide solution (10 mL). Subsequently, aqueoushydrogen peroxide solution (30%, 10 mL) was added dropwise at 0° C.within 15 minutes and the reaction mixture was heated to reflux for 3hours. Upon cooling to 20° C., the aqueous layer was saturated withpotassium carbonate and the organic layer was removed. The aqueous layerwas then extracted with ethyl acetate (3×20 mL). The combined organicphases were dried over anhydrous magnesium sulfate, filtered, andconcentrated to dryness. The crude product was dissolved indichloromethane (60 mL) and stored at 4° C. for overnight. Theprecipitate was filtered and the filtrate and washing were evaporated todryness. The residue was purified via column chromatography using a 9:1ethyl acetate to hexane mixture to afford the product as a clear liquid(1.1 g) in 83 percent yield. The molecular weight (MW) was 148.28.

EXAMPLE 2

Synthesis of a silyl-containing polyurethane thermoset—A thermoset witha 1:1.2 equivalent ratio of hydroxyl (OH) to isocyanate (NCO) groups wasmade by dissolving 0.74 g (0.010 equivalents of OH) of2,2′-(dimethylsilanediyl)bis(ethan-1-ol) (from Example 1) and 2.31 g(0.012 equivalents of NCO) of a trifunctional aliphatic isocyanate basedon hexamethylene diisocyanate, Desmodur N 3300A (Covestro), intetrahydrofuran (THF) at room temperature using a round bottom flask.The flask was heated and stirred in an oil bath at 50° C. for 1 hour,then poured into an aluminum weigh boat and cured in a 60° C. ovenovernight. A catalyst was not added. The thermoset was clear, as seen inFIG. 8. The thermoset gel fraction was 0.92, and the material had aglass transition temperature (T_(g)) of 68.1° C.

EXAMPLE 3

Disassembly of a silyl-containing polyurethane thermoset—The thermosetin Example 2 was placed in a vial and a 0.5 M solution oftetrabutylammonium fluoride (TBAF) in THF (Sigma-Aldrich) was added. Thethermoset remained static (no stirring) for a period of 1 week, thenremoved and dried in a vacuum oven. After 1 week, the glass transitiontemperature (T_(g)) of the thermoset had decreased to 35.3° C., and thethermoset was soft and rubbery.

EXAMPLE 4 (COMPARATIVE)

Non-silyl-containing polyurethane thermoset—A 1:1.2 equivalent ratio ofhydroxyl (OH) to isocyanate (NCO) groups was made by dissolving 1.04 g(0.020 equivalents of OH) of 1,5-pentane-diol (available fromSigma-Aldrich) and 4.62 g (0.024 equivalents of NCO) of a trifunctionalaliphatic isocyanate based on hexamethylene diisocyanate, Desmodur N3300A (Covestro), in tetrahydrofuran (THF) at room temperature using around bottom flask. The flask was then heated and stirred in an oil bathat 50° C. for 1 hour, then poured into an aluminum weigh boat and curedin a 60° C. oven overnight. A catalyst was not added. The thermoset wasclear with a gel fraction of 0.97, and the material had glass transitiontemperature (T_(g)) of 40.7° C. The thermoset was then placed in a vialand a 0.5 M solution of tetrabutylammonium fluoride (TBAF) in THF wasadded. The thermoset remained static (no stirring) for a period of 1week, then removed and dried in a vacuum oven. After 1 week, the glasstransition temperature (T_(g)) remained at 40.8° C., showing that thethermoset was unable to disassemble without the silyl triggers.

Obviously, many modifications and variations are possible in light ofthe above teachings. It is therefore to be understood that the claimedsubject matter may be practiced otherwise than as specificallydescribed. Any reference to claim elements in the singular, e.g., usingthe articles “a”, “an”, “the”, or “said” is not construed as limitingthe element to the singular.

What is claimed is:
 1. A thermoset made by a process comprising:providing a polyfunctional compound selected from polyisocyanate,polyacrylate, and polyepoxy; and reacting the polyfunctional compoundwith a hydroxyl- or amine-terminated silyl-containing compound;  whereinthe polyfunctional compound and the silyl-containing compound are atleast difunctional;  wherein the silyl-containing compound has theformulaSiR¹ _(n)[R³—(O—CO—X—R³)_(m)—X′]_(4-n);  wherein each X is independentlyselected from —O— and —NR²;  wherein each X′ is independently selectedfrom —OH and —NHR²;  wherein each R¹ is independently selected fromalkyl groups and aryl groups;  wherein each R² is independently selectedfrom —H, alkyl groups, and aryl groups;  wherein each R³ is anindependently selected alkylene group;  wherein n is 0, 1, or 2; and wherein each m is an independently selected positive integer.
 2. Thethermoset of claim 1, wherein the polyfunctional compound is apolyisocyanate, an aliphatic polyisocyanate, an aromatic polyisocyanate,a cycloaliphatic polyisocyanate, an isocyanate homopolymer, or anisocyanate-functional pre-polymer.
 3. The thermoset of claim 1, whereinthe polyisocyanate is an aliphatic trimer based on hexamethylenediisocyanate.
 4. The thermoset of claim 1, wherein the polyfunctionalcompound is an epoxy-functional dimethylpolysiloxane, anepoxy-functional polydimethyldiphenylsiloxane, an aliphatic epoxy, anaromatic epoxy, a cycloaliphatic epoxy, an acrylate-functionaldimethylpolysiloxane, or 1,6-hexanedioldiacrylate.
 5. The thermoset ofclaim 1, wherein the silyl-containing compound isSi(Ph)₂(CH₂CH₂OCON(CH₃)CH₂CH₂OH)₂.
 6. The thermoset of claim 1, whereineach R³ group is ethylene or propylene.
 7. The thermoset of claim 1,wherein the polyfunctional compound is reacted with ahydroxyl-functional or amine-functional material with at least onehydroxyl or amine group.
 8. The thermoset of claim 1, wherein thethermoset comprises at least 5 wt. % of the silyl-containing compound.9. A method comprising: reacting the thermoset of claim 1 with afluoride salt, an acid, or a base to cleave the silicon-carbon bonds inthe thermoset.
 10. The method of claim 9; wherein the fluoride salt istetrabutylammonium fluoride, stannous fluoride, potassium fluoride, orsodium fluoride; wherein the acid is a Lewis acid or Bronsted acid; orwherein the base is a Lewis base or Bronsted base.
 11. A method ofmaking a thermoset comprising: providing a polyfunctional compoundselected from polyisocyanate, polyacrylate, and polyepoxy; and reactingthe polyfunctional compound with a hydroxyl- or amine-terminatedsilyl-containing compound;  wherein the polyfunctional compound and thesilyl-containing compound are at least difunctional;  wherein thesilyl-containing compound has the formulaSiR¹ _(n)[R³—(O—CO—X—R³)_(m)—X′]_(4-n);  wherein each X is independentlyselected from —O— and —NR²—;  wherein each X′ is independently selectedfrom —OH and —NHR²;  wherein each R¹ is independently selected fromalkyl groups and aryl groups;  wherein each R² is independently selectedfrom —H, alkyl groups, and aryl groups;  wherein each R³ is anindependently selected alkylene group;  wherein n is 0, 1, or 2; and wherein each m is an independently selected positive integer.
 12. Themethod of claim 11, wherein the polyfunctional compound is apolyisocyanate, an aliphatic polyisocyanate, an aromatic polyisocyanate,a cycloaliphatic polyisocyanate, an isocyanate homopolymer, or anisocyanate-functional pre-polymer.
 13. The method of claim 11, whereinthe polyisocyanate is an aliphatic trimer based on hexamethylenediisocyanate.
 14. The method of claim 11, wherein the polyfunctionalcompound is an epoxy-functional dimethylpolysiloxane, anepoxy-functional polydimethyldiphenylsiloxane, an aliphatic epoxy, anaromatic epoxy, a cycloaliphatic epoxy, an acrylate-functionaldimethylpolysiloxane, or 1,6-hexanedioldiacrylate.
 15. The method ofclaim 11, wherein the silyl-containing compound isSi(Ph)₂(CH₂CH₂OCON(CH₃)CH₂CH₂OH)₂.
 16. The method of claim 11, whereineach R³ group is ethylene or propylene.
 17. The method of claim 11,wherein the polyfunctional compound is reacted with ahydroxyl-functional or amine-functional material with at least onehydroxyl or amine group.
 18. The method of claim 11, wherein thethermoset comprises at least 5 wt. % of the silyl-containing compound.19. The method of claim 11, wherein the reaction is performed with acatalyst, a solvent, a pigment, or an additive.
 20. The method of claim11, wherein the polyfunctional compound and the silyl-containingcompound are applied to a substrate via spray, brush, roll, print, ordip method before a cross-linked thermoset is formed.